find p-states heuristic

penhoat__joel — Tue, 07 Apr 2020 08:42:12 GMT

Hi,

"Intel® 64 and IA-32 Architectures Software Developer’s Manual Volume 3 (3A, 3B, 3C & 3D): System Programming Guide" gives an example to determine the target p-state (see Example 14-1. Determine Target P-state From Hardware Coordinated Feedback).

The proposed heuristic is based the function "FindPstate".

Do you know an example of such function. The pseudocode would be helpful to understand such function.

best regards

joel penhoat

Example 14-1. Determine Target P-state From Hardware Coordinated Feedback

DWORD PercentBusy; // Percentage of processor time not idle.
// Measure “PercentBusy“ during previous sampling window.
// Typically, “PercentBusy“ is measure over a time scale suitable for
// power management decisions
//
// RDMSR of MCNT and ACNT should be performed without delay.
// Software needs to exercise care to avoid delays between
// the two RDMSRs (for example, interrupts).
MCNT = RDMSR(IA32_MPERF);
ACNT = RDMSR(IA32_APERF);
// PercentPerformance indicates the percentage of the processor
// that is in use. The calculation is based on the PercentBusy,
// that is the percentage of processor time not idle and the P-state
// hardware coordinated feedback using the ACNT/MCNT ratio.
// Note that both values need to be calculated over the same

// time window.
PercentPerformance = PercentBusy * (ACNT/MCNT);
// This example does not cover the additional logic or algorithms
// necessary to coordinate multiple logical processors to a target P-state.
TargetPstate = FindPstate(PercentPerformance);
if (TargetPstate ≠ currentPstate) {
SetPState(TargetPstate);
}
// WRMSR of MCNT and ACNT should be performed without delay.
// Software needs to exercise care to avoid delays between
// the two WRMSRs (for example, interrupts).
WRMSR(IA32_MPERF, 0);
WRMSR(IA32_APERF, 0);

topic find p-states heuristic in Software Tuning, Performance Optimization & Platform Monitoring

find p-states heuristic